FTIR Spectroscopic Study of Biogenic Mn

Geomicrobiology Journal, 22:207–218, 2005
c Taylor & Francis Inc.
Copyright ISSN: 0149-0451 print / 1362-3087 online
DOI: 10.1080/01490450590947724
FTIR Spectroscopic Study of Biogenic Mn-Oxide Formation
by Pseudomonas putida GB-1
Sanjai J. Parikh and Jon Chorover
Department of Soil, Water, and Environmental Science, The University of Arizona, Tucson,
Arizona 85721, USA
Biomineralization in heterogeneous aqueous systems results
from a complex association between pre-existing surfaces, bacterial cells, extracellular biomacromolecules, and neoformed precipitates. Fourier transform infrared (FTIR) spectroscopy was used
in several complementary sample introduction modes (attenuated
total reflectance [ATR], diffuse reflectance [DRIFT], and transmission) to investigate the processes of cell adhesion, biofilm growth,
and biological Mn-oxidation by Pseudomonas putida strain GB-1.
Distinct differences in the adhesive properties of GB-1 were observed upon Mn oxidation. No adhesion to the ZnSe crystal surface
was observed for planktonic GB-1 cells coated with biogenic MnOx ,
whereas cell adhesion was extensive and a GB-1 biofilm was readily
grown on ZnSe, CdTe, and Ge crystals prior to Mn-oxidation. IR
peak intensity ratios reveal changes in biomolecular (carbohydrate,
phosphate, and protein) composition during biologically catalyzed
Mn-oxidation. In situ monitoring via ATR-FTIR of an active GB-1
biofilm and DRIFT data revealed an increase in extracellular protein (amide I and II) during Mn(II) oxidation, whereas transmission
mode measurements suggest an overall increase in carbohydrate
and phosphate moieties. The FTIR spectrum of biogenic Mn oxide comprises Mn-O stretching vibrations characteristic of various
known Mn oxides (e.g., “acid” birnessite, romanechite, todorokite),
but it is not identical to known synthetic solids, possibly because of
solid-phase incorporation of biomolecular constituents. The results
suggest that, when biogenic MnOx accumulates on the surfaces
of planktonic cells, adhesion of the bacteria to other negatively
charged surfaces is hindered via blocking of surficial proteins.
Keywords
FTIR spectroscopy, biomineralization, Mn oxidizing bacteria, Pseudomonas putida, Mn(IV) oxides, bacterial
adhesion
Received 16 September 2004; accepted 18 January 2005.
We thank Dr. Bradley M. Tebo and Brian Clement for donation
of GB-1 cells and providing information critical for cell growth and
Mn-oxidation. We also thank Martha Conklin for useful discussions at
early stages of this research and Hanna L. Gilbert for her assistance with
SEM-EDS analysis. TEM-EDS assistance and analysis was provided
by Sunkyung Choi and David Bentley. This research was supported
by the National Science Foundation CRAEMS program (Grant CHE0089156).
Address correspondence to Jon Chorover, Department of Soil,
Water, and Environmental Science, University of Arizona, Tucson,
AZ 85721, USA. E-mail: chorover@cals.arizona.edu
INTRODUCTION
Manganese is the second most abundant transition metal in
the earth’s crust behind Fe and, like Fe, its oxidation-reduction
reactions are largely mediated by biological activity (Lovley
2000). Microbial catalysis is known to accelerate the kinetics
of Mn(II) oxidation and promote the formation of Mn(IV) oxide minerals in natural waters (Nealson et al. 1988; Tebo et al.
1997). Manganese oxides (MnOx ) are produced biogenically by
numerous species of bacteria, including Pseudomonas putida
strain GB-1, a fresh-water, facultative-aerobic, gram-negative
bacteria. The formation of MnOx can influence the environmental fate of other metals (e.g., Cu, Co, Cd, Zn, Ni, and Pb) through
co-precipitation and adsorption reactions (Nelson et al. 1999,
2002; Tani et al. 2003, 2004; Tebo et al. 2004). The physiological basis for bacterially-mediated Mn oxidation is not known, but
it is thought that biogenic MnOx may serve to protect cells from
Mn or other metal toxicity (e.g., heavy metals), UV irradiation,
or other potential threats (Brouwers et al. 2000).
The identity of biogenic MnOx phases is diverse with apparent dependence on the type of microbial catalyst and conditions
of formation. For example, Mandernack et al. (1995) reported
mixed phase minerals (hausmmannite, Mn3 O4 ; feiknechtite, βMnOOH; manganite, γ -MnOOH, and Na-buserite) following
Mn(II) oxidation by a marine Bacillus strain SG-1, whereas a
nanocrystalline todorokite-like mineral was produced by Leptothrix discophora strain SP-6 (Kim et al. 2003). The MnOx
formed by P. putida strain MnB1 was found to be most similar
to “acid” birnessite (Villalobos et al. 2003), whereas Mn oxide
crusts from Pinal Creek, AZ comprise a mixture of todorokite
and birnessite or takanelite/ranciete, possibly deriving from a
buserite precursor (Bilinski et al. 2002; Gilbert 2003).
The mechanisms of biotic Mn(II) oxidation and the subsequent binding of MnOx to cell surfaces are not known. One or
more enzymes is likely responsible for catalyzing Mn(II) oxidation. Recent research indicates that the cumA gene, a multicopper
oxidase (Brouwers et al. 1999; Francis and Tebo 2001), and/or
a general secretion pathway (xcp in Pseudomonas species) gene
(de Vrind et al. 2003) are integrally involved in Mn oxidation. However, these genes have not been identified unambiguously. Although great progress has been made in identifying
207
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S. J. PARIKH AND J. CHOROVER
the enzymes involved in Mn oxidation (Tebo et al. 2004), the
specific macromolecules (e.g., protein or protein/carbohydrate
complex) have not been determined (de Vrind et al. 1998).
Upon oxidation by GB-1, MnOx are precipitated on the cell
surface in intimate association with surficial macromolecules
(Okazaki et al. 1997). These surface macromolecules may serve
to template the oxide nucleation process and influence subsequent crystal growth in a manner that has been observed, for
example, with Fe oxyhydroxides (Nesterova et al. 2003; Chan
et al. 2004). When MnOx encrustations are formed on the surface of planktonic cells, they are expected to alter the cell surface
charge and hydrophobicity, thus altering adhesion and transport
properties of the cells (Rosenberg and Kjellberg 1986; van Loosdrecht et al. 1987; van der Mei et al. 1989). The solids may
also be formed by intact biofilms that are already attached to
stable substrata. The process of biogenic MnOx formation in
heterogeneous aqueous systems may, therefore, be considered
a ternary interaction among bacterial cells, neoformed MnOx ,
and existing surfaces (substrata) for biofilm growth. The favorability of these various interactions is governed to a large degree by the respective surface chemistries. The charge and hydrophobicity of bacterial cells are affected by the composition
of surficial macromolecules including extracellular polymeric
substances (EPS; i.e., extracellular polysaccharides, proteins,
and nucleic acids) (Wingender et al. 1999), lipopolysaccharides
(LPS) (Williams and Fletcher 1996), and surface appendages
such as fimbrae (Scannapieco et al. 1983). The degree to which
biomolecular composition may be altered in response to environmental cues—such as availability of dissolved Mn(II) and
the formation of MnOx —is not well known, but it is expected to
influence the molecular-scale mechanisms of mineral-microbe
interaction.
An improved understanding of the process of biogenic MnOx
formation should follow from in situ investigation into the production, composition, and structure of biogenic MnOx and the
co-evolving biomolecular matrix, referred to hereafter as
GB-1/MnOx . Fourier transform infrared (FTIR) spectroscopy is
well-suited for such studies, because it provides simultaneously
molecular-scale information on both organic and inorganic constituents of a sample. FTIR spectroscopy uses polychromatic radiation to measure the excitation of molecular bonds whose relative absorbances provide an index of the abundance of various
functional groups (Griffiths and de Haseth 1986). Since it can be
used to probe distinct vibrations arising from both biomolecules
and inorganic solids, it is emerging as a useful tool for investigating processes at the bacteria-mineral and biomolecule-mineral
interface (Deo et al. 2001; Benning et al. 2004; Omoike et al.
2004). In this work, FTIR spectroscopy was used to determine
whether Mn(II) oxidation results in a detectable change in the
biomolecular composition of GB-1 suspensions and biofilms.
Since prior observations suggest a strong association between
P. putida EPS and neoformed biogenic Mn oxides (Okazaki et
al. 1997), we hypothesized that Mn biomineralization results in
IR detectable changes in the relative proportion of polysaccha-
ride and protein constituents external to the cell. Complementary
modes of FTIR sample introduction were employed to resolve
changes in molecular composition of the mineral-microbe interface relative to those occurring in the bulk.
EXPERIMENTAL METHODS
Bacterial Strain, Media, and Growth Conditions
Pseudomonas putida strain GB-1 was generously provided
by B. M. Tebo (Scripps Institute of Oceanography). Bacteria
were plated on Leptothrix discophora agar (Boogerd and de
Vrind 1987) agar at pH 7.5 with 0.2 mM MnSO4 to ensure
the Mn oxidizing factor was present. Bacteria were grown in
250 mL polycarbonate screw cap Erlenmeyer flasks (Nalgene),
and subjected to orbital mixing at 100 rpm in an environmental shaker at 30◦ C. The growth medium contained the following concentrations of mineral salts, trace elements, and glucose (MSTG) dissolved in Barnstead nanopure water: 2 mM
(NH4 )2 SO4 , 0.25 mM MgSO4 , 0.4 mM CaCl2 , 0.15 mM
KH2 PO4 , 0.25 mM Na2 HPO4 , 10 mM HEPES, 0.01 mM FeCl3 ,
0.01 mM EDTA, 1 mM glucose, and 1 mL of trace metal solution (10 mg/L CuSO4 · 5H2 O, 44 mg/L ZnSO4 · 7H2 O, 20 mg/L
COCl2 · 6H2 O, and 13 mg/L Na2 MoO4 · 2H2 O). Prior to autoclaving, the pH was adjusted to 7.4 with 2 M NaOH. Solutions of
FeCl3 and trace metals were filter sterilized (0.2 µm) and added
to autoclaved MSTG media. For Mn oxidation experiments,
MnSO4 (0.2 mM) was added to MSTG media. In the study of
adhesion of GB-1 cells to transmission windows, Rein’s vitamin
solution (for 200 mL: 40 mL of 1 mg mL−1 biotin, 4 mg nicotinic
acid, 2 mg, thiamin, 4 mg p-aminobenzoic acid, 2 mg calcium
pantothenate, 20 mg pyridoxine, 2 mg cyannobalamin, 4 mg riboflavin, and 4 mg folic acid) was used (1 mL L−1 ) instead of the
trace metal solution (mineral salts, vitamins, glucose; MSVG).
A leucoberbeline blue (LBB) assay was used to confirm Mn oxidation (Boogerd and de Vrind 1987). Growth of all bacteria was
carried out with autoclaved materials under pure culture conditions. Growth curves were determined from UV absorbance
(λ = 600 nm) of cell suspensions using replicate samples.
Electron Microscopy
GB-1 cells were harvested from the late exponential phase
(24 h) and stained using 0.5% ammonium molybdate (pH 7.56).
Samples were dried onto carbon and piloform coated copper
grids and observed at 200 kV with a Hitachi H8100 LaB6 transmission electron microscope (TEM) equipped with energy dispersive X-ray spectroscopy (EDS) and electron diffraction (ED).
GB-1/MnOx collected from the stationary phase, were also examined without the negative stain to observe electron dense
MnOx coatings. EDS was performed to verify the composition of Mn coatings. GB-1 biofilms on transmission windows
were analyzed with a Hitachi S-2460N natural scanning electron microscope (SEM) with EDS in NatureMode (∼40 Pa,
25 kV, working distance 21 mm) with a back-scattered electron
Robinson detector.
FTIR STUDY OF BIOGENIC Mn OXIDATION
209
FIG. 1. Schematic illustration demonstrating the basic concepts of: (a) transmission spectroscopy, (b) DRIFT spectroscopy, (c) ATR (ARK) spectroscopy, and
(d) ATR (circle cell) spectroscopy (d.p. = depth of penetration).
Methods of Sample Introduction for FTIR Spectroscopy
All FTIR spectra were collected using a Nicolet 560 Magna
IR spectrometer (Madison, WI). Transmission, diffuse
reflectance, and attenuated total reflection sample introduction
techniques was employed to characterize the bulk and extracellular composition of GB-1 and GB-1/MnOx (Figure 1). Each
technique has inherent strengths and weaknesses, and provides
complementary data. Transmission mode (Figure 1a) provides
data on the bulk sample (e.g., whole cell). One disadvantage of
transmission mode is the need for sample desiccation, possibly
creating artifacts, such as the dehydration of surface complexes.
In diffuse reflectance FTIR (DRIFT; Figure 1b) radiation penetrates the sample to a depth that depends on the reflective and
absorptive characteristics of the sample. Re-emision of this radiation occurs in all directions (i.e., “diffusely”). Upon collection, the emitted radiation produces an “absorbance spectrum”
that is comparable to that produced in transmission mode, but
more dependent on the spectral properties of the sample interface (Griffiths and de Hasseth 1987).
Attenuated total reflectance (ATR) FTIR spectroscopy
(Figure 1c, 1d) provides nondestructive, in situ information on
aqueous phase samples, including microbial cells and biofilms
(Nichols et al. 1985; Nivens et al. 1993; Schmitt et al. 1995;
Schmitt and Flemming 1998). This technique interrogates IRabsorbing functional groups of the sample that reside in close
proximity (ca. ≤1 µm) to the interface with an IR-transparent
internal reflection element (IRE) (Nivens et al. 1993). ATRFTIR techniques are particularly useful for examining changes
in amide, carbohydrate and other polar functional groups resulting from biofilm evolution (e.g., Nichols et al. 1985), biomolecular composition/conformation (e.g., Omoike and Chorover 2004)
and surface complexation (Omoike et al. 2004). As a result of
the limited beam penetration depth, ATR collects information
on the composition of the bacteria-IRE interface, giving rise to
spectra that emphasize the molecular composition of bacterial
surfaces. While the reflected beam may penetrate into cells if
the outer cell membrane is adhered directly to the crystal surface, this effect is diminished when EPS is interposed between
cells and the IRE. The acquired spectra thus contain peaks deriving from vibrational modes of both surficial and internal cell
material but, relative to transmission data, there is clearly bias
toward surface composition. ATR data were collected either in
batch mode (e.g., using the ARK cell; Figure 1c) or flow-through
mode (e.g., using the cylindrical circle cell; Figure 1d).
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S. J. PARIKH AND J. CHOROVER
Transmission FTIR Spectroscopy of GB-1/MnOx
GB-1 cells were grown in both the presence and absence of
MnSO4 (MSTG media) and harvested at 24 h. A 250 mL volume
of cell suspension was centrifuged at 5,000 relative centrifugal
force (RCF) for 20 min (4◦ C) and cells were concentrated to a
10 mL volume. Then 100 µL of sample was transferred onto
IR transmission windows (ZnSe, CdTe, or Ge) and dried under vacuum (340 mbar) overnight. The accessible wavenumber
ranges for ZnSe, CdTe, and Ge windows are 20,000 to 454 cm−1 ,
20,000 to 360 cm−1 , and 5500 to 475 cm−1 respectively; with
the CdTe crystal being more effective for data collection in the
Mn-O stretching region (750 to 200 cm−1 ; Julien et al. 2004).
Transmission spectra were collected with a minimum of 400
scans at a 4 cm−1 resolution.
Diffuse Reflectance Infrared Fourier Transform (DRIFT)
Spectroscopy of GB-1/MnOx
GB-1 cells were grown both in the presence and absence of
MnSO4 (MSTG media) and harvested in the stationary phase
(24 h). GB-1 and GB-1/MnOx were washed twice with 0.1 M
NaCl (pH 7.0) by vortex mixing, shaken for 60 s, and then centrifuged (5000 RCF, 20 min, 4◦ C). Pelleted cells were lyophilized
and then diluted with KBr to approximately 10% (w/w) by gently
mixing 39 mg of sample with 30 mg of KBr for 40 s, then folding in an additional 390 mg of KBr to homogenize the samples.
DRIFT spectra were collected with a minimum of 400 scans at
4 cm−1 resolution (Figure 1b).
DRIFT spectra were also collected on washed GB-1/MnOx
samples generated from cells grown in the presence of MnSO4
and harvested at 24 h. The washing procedure, which is a modification of more aggressive methods that employ organic solvents
and/or oxidants (Mandernack et al. 1995; Villalobos et al. 2003),
was intended to reduce biomass concentration without altering
the MnOx structure. In this case, the use of oxidants and solvents was avoided. GB-1/MnOx were centrifuged and washed
twice with 0.1 M NaCl (pH 7.0, 25,000 RCF, 20 min, 4◦ C).
GB-1/MnOx were resuspended in 0.1 M NaCl by vortex mixing
and sonicated for 30 min. The MnOx mixture was then washed an
additional five times (0.1 M NaCl, pH 7.0, 25000 RCF, 20 min,
4◦ C). After each wash procedure, the organic material that had
settled onto the MnOx was physically removed with a spatula
via gentle scraping. Washed MnOx samples were freeze-dried
and the lyophilized material was again subjected to removal of
any visible organic matter. Samples (5% w/w) were prepared
and analyzed by DRIFT as described above. Spectra of acid
birnessite were also collected by DRIFT. Acid birnessite was
synthesized by adding concentrated HCl to a boiling solution of
KMnO4 (McKenzie 1971).
Attenuated Total Reflectance (ATR) FTIR Spectroscopy
of GB-1/MnOx
GB-1 cells were grown in MSTG media and harvested at 24
h. Cells were concentrated as described for transmission FTIR,
transferred (1 mL) to a ZnSe ARK internal reflection element
(IRE), and allowed to settle onto the IRE for 4 h. Spectra were
collected with a minimum of 400 scans at a 4 cm−1 resolution using the growth medium as background for subtraction
(Figure 1c).
Cell Adhesion to FTIR Transmission Windows
The adhesion of GB-1 cells to IR-transparent crystal surfaces
was assessed in the presence and absence of MnSO4 (MSVG media) by conducting two separate batch experiments, each with
three types of window materials (ZnSe, Ge, and CdTe) suspended in cell culture suspensions. A Virtis Omni Culture Plus
bioreactor (Gardiner, NY) was used to maintain constant temperature (30◦ C), pH (7.5), mixing (100 rpm), and aeration over
the course of the experiment. The bioreactor (1.8 L) was inoculated with 15 mL of preculture harvested at the early stationary phase of growth. Influx of fresh media to the bioreactor
(beginning at 24 h) was equivalent to cell suspension efflux
(0.25 mL/min). Transmission windows were suspended in the
bioreactor for the duration of the experiment (85 h). Adsorbed
biofilms were dried on the same windows where they formed,
and spectra were collected in transmission mode as described
previously. Biofilm-coated transmission windows were then analyzed by SEM-EDS. FTIR peak ratios were determined using
maximum IR absorbance values for specified wavenumbers.
In situ Monitoring of Biofilm Growth and Mn Oxidation
Cell adhesion, biofilm growth, and the effects of Mn oxidation were studied in real-time using a flow-through ATR-FTIR
method. The Virtis bioreactor (growth conditions as described
for transmission FTIR studies of cell adhesion) containing 1.8 L
of MSTG media was inoculated with 15 mL of GB-1 preculture in the absence of MnSO4 . For the first 24 h, solution was
pumped out of the bioreactor, through the flow cell (Figure 1d;
ZnSe ATR IRE), and then recirculated back into the bioreactor.
At 24 h fresh media was added into the bioreactor as cells passing the flow cell were pumped to waste (0.25 mL/min). Once a
stable biofilm spectrum was observed (79 h), MnSO4 (200 µM)
was added by mixing to combine with the cell suspension (ca.
109 cells/mL) at a Y connection placed into the effluent tube of
the bioreactor. The combined cell suspension/MnSO4 was then
introduced into the ATR-FTIR flow cell (with ZnSe IRE) and
out to waste (Figure 1d). Using this approach, no MnSO4 was
introduced into the bioreactor and all Mn(II) oxidation occurred
downstream of the Y connection. The flow rate was maintained
at 0.25 mL/min, and spectra were collected as a function of
time during flow. This experimental design probes the effects
of Mn oxidation by cells adhered to the IRE in the FTIR flow
cell, therefore making it possible to observe resultant changes in
the biofilm. Spectra were corrected for growth media as background. In situ flow through experiments were also conducted
in the absence of Mn(II), and for cells actively oxidizing Mn in
the bioreactor prior to introduction to the FTIR flow cell/ZnSE
IRE.
FTIR STUDY OF BIOGENIC Mn OXIDATION
RESULTS
Cell Growth, Biogenic MnOx Production,
and TEM Analysis
Growth curves of GB-1 indicate the early stationary phase
is reached at 16 h in MSTG and 24 h in MSVG. When using
MSVG, Mn(II) oxidation is first observed around 20 h, and with
MSTG the time is approximately 12 h. The presence of MnOx
was confirmed through a positive reaction to the LBB assay.
Addition of MnSO4 to cells in the late exponential phase leads
to Mn oxidation and formation of MnOx aggregates.
GB-1 cells required negative staining for TEM observation.
Micrographs acquired on GB-1 cells comprise ammonium
molybdate-stained GB-1 cells with flagella (not shown). TEM of
GB-1 cells with MnOx coatings precipitated on cell surfaces did
not require stain, due to high electron density of the Mn-coatings
(Figure 2, inset a). The presence of Mn in the solids is evident
from EDS results (Figure 2; EDS spot size is 100 nm). The data
also indicate the presence of K, Ca, and Na in the biogenic solids,
211
which is characteristic of naturally occurring birnessites. Electron diffraction patterns (Figure 2 inset b; ED spot size is 1 µm)
indicate that the biogenic MnOx exhibits poor crystallinity relative to synthetic analogs (Drits et al. 1997; Bilinski et al. 2002),
consistent with the X-ray diffraction studies of Villalobos et al.
(2003).
IR Spectral Characterization of GB-1 and GB-1/MnOx
Transmission Spectroscopy
Spectra of GB-1 cells dried onto ZnSe and CdTe windows
(Figure 1a) are shown in Figure 3. The main difference between
GB-1 and GB-1/MnOx is observed on the CdTe window, with
peaks at 431, 418, 408, 399, 377, and 369 cm−1 , corresponding
to IR absorbances of MnOx (Julien et al. 2004). Unlike ZnSe,
the use of CdTe crystals allows spectra to be collected to low
wavenumbers (i.e., 360 cm−1 ) where Mn-O stretching occurs. A
spectral downshift to lower wavenumbers of several biomolecular peaks is observed in the presence of MnOx (e.g., 1652 to
FIG. 2. EDS Spectrum of P. putida GB-1/MnOx harvested in the late exponential phase. Inset a) TEM micrograph of unstained GB-1/MnOx (bar = 0.5 µm);
inset b) Electron diffraction pattern.
212
S. J. PARIKH AND J. CHOROVER
TABLE 1
Pertinent IR assignments for GB-1 and GB-1/MnOx systems
Wavenumber (cm−1 )
1652–1637
1550–1540
1460–1454
1400–1390
1316–1306
1240
1220
1168
1112–1114
1126–1119, 1085,
1040–1035, 1020
750–200∗
650–450
IR band assignment
Amide I: C O, C N, N Ha-c
Amide II: N H, C Na-c
C H: CH2 (scissor)a,c
C O: vs (COO− )a,c
Amide III: C N, N H, C Oc
Metal-complexed P Oa,e
Hydrated P Oa,e
v(C O)d
C O P, P O P, ring vibrationsa,d
C O C, C C, C O ring vibrations
(sugars)a
MnOx stretching, bending, and
wagging vibrationsf
CH2 vibrations of polysaccharidesg
∗
750–600, 600–450, and 450–200 correspond to Mn-O stretching,
bending, and wagging, respectively.
a
Schmitt and Flemming (1998), b Sockalingum et al. (1997), c Nivens
et al. (1993), d Brandenburg and Seydel (1996), e Brandenburg et al.
(1997), f Julien et al. (2004), g Deo et al. (2001).
FIG. 3. Transmission FTIR spectra of GB-1 cells dried onto ZnSe and CdTe
crystals.
1648, 1540 to 1535, 1039 to 1033, 1170 to 1166, 1085 to 1074).
FTIR assignments for GB-1 and GB-1/MnOx systems are provided in Table 1. A decrease in peak intensity at 1390 cm−1
(νsym , COO− ) is observed with Mn-oxidation, as is the formation of a doublet (at 1658 and 1648 cm−1 ) from the amide I peak
(at 1652 cm−1 in the absence of MnOx ).
IR spectral effects of MnOx were quantified using peak intensity ratios determined by comparison of maximum IR absorbance values at selected wavenumbers (Niemeyer et al. 1992;
Ishida and Griffiths 1993; Wander and Traina 1996; Gallé et al.
2004). Values appear to be independent of the type of transmission window and the amide II (1540 cm−1 ) / amide I (1650 cm−1 )
ratio is not affected by Mn oxidation (Table 2). The increased ratio of both phosphate (1220cm−1 ; vas P O of phosphodiesters)
and carbohydrate (1168, 1120, 1085, and 1035 cm−1 ) to amide I
indicate a relative increase in polysaccharides relative to proteins
(amide I and II) with Mn oxidation.
DRIFT Spectroscopy
Spectra collected on lyophilized GB-1 and GB-1/MnOx in
DRIFT mode (Figure 4) are similar to the transmission spectra
(Figure 3). However, unlike transmission results, DRIFT spectra show a decrease in the ratio of carbohydrate (1085 cm−1 )
to amide I (Table 2) for biomineralized systems. A large Mn-O
stretching peak is also observed at 433 cm−1 for GB-1/MnOx .
The similarity of DRIFT spectra for GB-1/MnOx before and
after washing (Figure 4) indicates that the organic components
of the GB-1/MnOx complex is resistant to desorption and removal by the relatively mild treatment employed; both spectra
exhibit prominent biomolecular peaks. The spectral subtraction
of GB-1 from GB-1/MnOx gives the difference spectrum of biogenic MnOx shown in Figure 5. This spectrum, which shows
Mn-O vibrations at 349, 323, 302, and 271 cm−1 (Julien et al.
2004) is comparable, but not identical to, that of acid birnessite (shown for comparision in Figure 5) that was synthesized
abiotically in the laboratory according the method of McKenzie
(1971).
ATR (ARK) Spectroscopy
ATR (ARK, Figure 1c) spectra for GB-1 and GB-1/MnOx
(Figure 6) show an increase in amide I relative to carbohydrate,
phosphate, and even amide II absorbances in the presence of
MnOx (Table 2).
Adhesion to Transmission Windows
Transmission spectra of GB-1 biofilms adhered from cell culture suspension to IR windows are similar for all crystal types
(ZnSe, Ge, and CdTe) (Figure 7). SEM analyses showed a comparable extent of cell adhesion regardless of crystal material and
EDS confirmed the presence of Mn particles in biofilms grown
with MnSO4 (not shown). Spectral differences are observed,
however, between GB-1 cells that oxidized Mn and those that
did not. There was a downshift in wavenumber of the P O absorbance with Mn oxidation (1238 cm−1 to 1220 cm−1 ), which
213
FTIR STUDY OF BIOGENIC Mn OXIDATION
TABLE 2
FTIR absorption ratios via various sample introduction techniques
Wavenumber ratio∗
IR method
Transmission
DRIFT
ATR (ARK)
∗
IR window
and sample
ZnSe: GB-1
ZnSe:GB-1
with MnOx
CdTe: GB-1
CdTe: GB-1
with MnOx
GB-1
GB-1 with
MnOx
GB-1
GB-1 with
MnOx
vamII :vam I
1540:1650
vP O :vam I
1240:1650
vP O :vam I
1220:1650
vcarb :vam I
1170:1650
vcarb :vam I
1120:1650
vcarb :vam I
1085:1650
vcarb :vam I
1035:1650
0.68
0.71
—
—
0.71
0.89
0.75
1.0
0.61
0.81
0.63
0.96
0.73
1.3
0.75
0.69
—
—
0.73
0.84
0.72
1.0
0.67
0.77
0.72
0.92
0.75
1.4
0.69
0.69
0.27
0.22
—
—
0.03
0.06
—
—
0.25
0.18
—
—
1.0
0.82
0.66
0.52
—
—
0.33
0.26
0.62
0.49
1.0
0.81
—
—
Wavenumber values are approximate, and exact peak location varies between spectra.
is accompanied by a relative increase in phosphate absorbance
intensity (Figure 7). Peak shifts of similar magnitudes have been
observed upon inner-sphere complexation of phosphate at Feoxide surfaces (Tejedor-Tejedor and Anderson 1986, 1990; Arai
and Sparks 2001) and with dissolved Mg2+ (Brandenberg et al.
1997). In the present case, this shift may result from phosphate
bonding to Fe or Mn bearing surfaces.
The intensity ratio of phosphate to amide I increased in the
presence of MnOx (Table 3). A relative increase in peak intensity at 1038 cm−1 indicates polysaccharide enrichment. Spectra
FIG. 4. DRIFT Spectra of GB-1, GB-1/MnOx , washed GB-1/MnOx , and synthetic acid birnessite.
FIG. 5. Subtraction result of DRIFT spectra for GB-1/MnOx minus GB-1 and
spectra of synthetic acid birnessite.
214
FIG. 6.
S. J. PARIKH AND J. CHOROVER
ATR-FTR spectra of GB-1 cells deposited on a ZnSe (ARK) IRE.
collected using the CdTe window reveal a small peak at 588
cm−1 , attributed to Mn-O stretching vibrations. Although Mn
is detected in biofilms (via EDS) and brown precipitates on
biofilms were visible, the concentration of MnOx is evidently
too low to give IR absorption peaks comparable to those observed for the more concentrated samples dried onto transmission windows (GB-1 cells deposited and dried onto transmission
windows).
In situ Monitoring of GB-1 Biofilm Growth
and Mn Oxidation
When GB-1/MnOx (cells already oxidizing Mn in the bioreactor) were passed over a clean ZnSe IRE (Figure 1d), no adhesion or biofilm growth was detected even after >65 h of reaction.
However, in the absence of Mn, adhesion and biofilm growth
(evident from a time-dependent increase in IR absorbances of
biomolecular constituents) were detectable at much earlier times
(Figure 8). Bacteria adhere to the ZnSe crystal surface early in
the experiment, and a mature biofilm is formed well after the
cell suspension enters the stationary phase (>50 h). Upon addition of MnSO4 (79 h) a dramatic increase in the IR intensity
for major biofilm peaks was observed (t = 88 h) (Figures 8
and 9). MnOx particles were visible in the flow cell approximately 9 h after MnSO4 addition. Biofilm growth may be quantified on the basis of time-dependent changes in absorbance
of protein regions of the IR spectrum (Figure 9). Addition of
MnSO4 resulted in a ca. 100% increase in absorbance intensity
for both amide I and amide II (increasing from approximately
FIG. 7. Transmission FTIR spectra of GB-1 biofilms on ZnSe, Ge, and CdTe
crystals suspended in bioreactor.
0.015 to 0.033 absorbance units for both the amide I and II). In a
MnSO4 -free control (no MnSO4 added), maximum absorbance
values for the amide peaks remained lower, even at long times
(137 h; Abs(amide I) = 0.0198 and Abs(amide II) = 0.0204).
Time-dependent changes in IR absorbance ratios for GB-1
biofilm growth and subsequent Mn oxidation are shown in Figure
10. The trends include a relative decrease in amide II, phosphate
(1240 cm−1 ) and polysaccharide (1166 and 1085 cm−1 ) peaks
relative to amide I over the reaction time. Although the plotted
ratios all decrease prior to addition of Mn(II), the rate of decrease
is faster after addition of Mn(II) (Table 4). A large increase
in protein relative to other biomolecular constituents following
MnSO4 addition is, therefore, evident from the real-time ATR
studies.
DISCUSSION
Bulk Versus Extracellular Composition
In contrast to the transmission results (Figures 3 and 7), which
show enrichment in phosphate and polysaccharide relative to
protein during Mn(II) oxidation, ATR (Figure 6) data indicate
a relative increase in protein (Table 2). This apparent discrepancy in effects of Mn(II) oxidation on biomolecular composition is resolved by considering the fact that ATR methods
215
FTIR STUDY OF BIOGENIC Mn OXIDATION
TABLE 3
IR absorption ratios: adhesion of GB-1 to transmission windows
Wavenumber ratio∗
IR Window and Sample
vamII :vam I
1540:1650
vP O :vam I
1238:1650
vP O :vam I
1220:1650
vcarb :vam I
1168:1650
vcarb :vam I
1080:1650
vcarb :vam I
1035:1650
ZnSe: GB-1
ZnSe: GB-1 with MnOx
CdTe: GB-1
CdTe: GB-1 with MnOx
Ge: GB-1
Ge: GB-1 with MnOx
0.66
0.61
0.67
0.60
0.62
0.56
0.37
—
0.42
—
0.37
—
—
0.41
—
0.47
—
0.38
0.29
0.42
0.30
0.48
0.31
0.37
0.44
0.53
0.46
0.50
0.41
0.43
0.43
0.59
0.43
0.59
0.41
0.43
∗
Wavenumber values are approximate, and exact peak location varies between spectra.
probe preferentially the surficial regions of the GB-1/MnOx
samples (cell/MnOx interface), whereas transmission mode is
a bulk (whole particle absorption) technique. Therefore, the
data suggest that Mn oxidation by GB-1 cells is accompanied
by an increase in bulk polysaccharide and phosphate, whereas
the extracellular environment is enriched in proteinaceous constituents. Evidently, the presence of Mn(II) in solution results in
an increased expression of extracellular proteins that promote
the biomineralization of MnOx .
Effect of Mn-Oxidation on Amide Groups
The decreased intensity ratio of amide II (1540 cm−1 )/amide
I (1650 cm−1 ) in ATR spectra (Figure 6, Table 2) indicates
that, although the overall protein content of GB-1 cells may
not change, the expression of surficial protein is indeed altered
upon Mn oxidation. Changes in the amide II / amide I intensity
ratio suggest variation in protein secondary structure (Ishida and
Griffiths 1993). The amide I ATR peak of GB-1 cells does not
shift upon Mn-oxidation (Table 5). Second derivative analysis
of GB-1 and GB-1/MnOx ATR spectra in Figure 6 both reveal
a protein mixture comprising β-sheet (1629 and 1637 cm−1 ),
random coil (1645 cm−1 ), and α-helical (1652 cm−1 ) conformations (Buijs et al. 1996). The amide I peak maximum for
hydrated samples (ATR) occurs at 1637 cm−1 , corresponding
to β-sheets (Table 5). A shift in the amide I region to higher
wavenumber in DRIFT and transmission spectra (Table 5) is
consistent with previous studies (Hübner and Blume 1988) indicating that this change reflects the impact of sample dehydration.
The presence of a small doublet peak in transmission spectra at
1658 (α-helical) and 1648 cm−1 (random coil/α-helical) (Buijs
et al. 1996; Jung 2000) suggest a change in protein conformation
due to Mn-oxidation/-binding (Figure 3, Table 5). The peak shift
from 1652 cm−1 (α-helical) represents an increase in randomly
coiled proteins after Mn-oxidation.
FIG. 8. ATR (circle cell) spectra for in situ monitoring of GB-1 adhesion,
biofilm growth, and Mn-oxidation on a ZnSe IRE.
FIG. 9. Amide I and II maximum IR absorbance as a function of time. MnSO4
was added to the flow cell at 79 h and visible MnOx were observed at 88 h.
216
S. J. PARIKH AND J. CHOROVER
TABLE 5
Location of maximum amide I IR absorbance
IR method
Transmission
DRIFT
ATR (ARK)
FIG. 10. Plot of IR peak intensity ratios versus time from GB-1 flow through
experiment. MnSO4 was added to the flow cell at 79 h. After addition of Mn(II)
a slope change is observed.
Characterization of Biogenic MnOx
The fact that DRIFT spectra of GB-1/MnOx show no effect
of repeated washing and density separation in electrolyte solution indicates a strong, intimate association between MnOx and
biomolecular material. Given that organic complexation of neoformed mineral colloids inhibits their crystallization, it is not
surprising that biogenic MnOx exhibits short-range crystal order (Mizukami et al. 1999; Villalobos et al. 2003). Transmission
(Figure 3, CdTe) and DRIFT spectra (Figure 5) of GB-1/MnOx
show numerous vibrations in the Mn-O stretching region. Since
IR absorbance of polysaccharides also occurs below 700 cm−1
(Deo et al. 2001), Mn-O absorbances are assessed after subtraction of bands arising from cell constituents. The MnOx DRIFT
difference spectrum (Figure 5) is comparable to those observed
for Mn oxides (e.g., birnessite, romanechite, todorokite) (Potter
and Rossman 1979; Julien et al. 2004). Based on extensive mineralogical analyses following more aggressive removal of bound
TABLE 4
Regression data from IR absorption ratio plot (Figure 10):
GB-1 growth and Mn-oxidation on ZnSe IRE (circle cell)
Carb: Am I (1085:1637)
0–79 h
89–120 h
P O: Am I (1240:1637)
0–79 h
89–120 h
Carb: Am I (1166:1637)
0–79 h
89–120 h
Regression equation
R2
Y = 1.82–0.011X
Y = 2.34–0.015X
0.94
0.99
Y = 1.40–0.009X
Y = 1.75–0.011X
0.97
0.99
Y = 1.07–0.007X
Y = 1.52–0.012X
0.99
0.99
Transmission:
GB-1 Adhesion
ATR (Circle Cell):
GB-1 Adhesion
1548 min
3000 min
4680 min
5214 min
6090 min
7230 min
IR window
and sample
Max. amide I (cm−1 )
ZnSe: GB-1
ZnSe:GB-1
with MnOx
CdTe: GB-1
CdTe: GB-1
with MnOx
GB-1
GB-1 with
MnOx
GB-1
GB-1 with
MnOx
1652
1658 & 1648
ZnSe: GB-1
ZnSe: GB-1
with MnOx
CdTe: GB-1
CdTe: GB-1
with MnOx
Ge: GB-1
Ge: GB-1
with MnOx
1656
1656
ZnSe: GB-1
ZnSe: GB-1
ZnSe: GB-1
ZnSe: GB-1
with MnSO4
ZnSe: GB-1
with MnOx
ZnSe: GB-1
with MnOx
1637
1635
1637
1637
1652
1658 & 1648
1656
1660
1637
1637
1654
1654
1654
1654
1637
1637
organic matter with phenol/chloroform and NaOCl, Villalobos
et al. (2003) reported that physicochemical properties of the biogenic MnOx produced by P. putida strain MnB1 are intermediate between synthetic vernadite (δ-MnO2 ) and randomly stacked
acid birnessite. The latter was synthesized in their study and the
present one using the HCl-induced KMnO4 reduction method
of McKenzie (1971). Differences between the FTIR spectra of
synthetic acid birnessite and GB-1 biogenic MnOx (Figure 5)
may be due to our less aggressive oxide cleaning procedure
and the associated preservation of mineral-bound biomolecular
material. As noted by Villalobos et al. (2003), aggressive removal of cellular material may affect the structure of the MnOx
product, but its retention in the solid hinders a direct comparison
FTIR STUDY OF BIOGENIC Mn OXIDATION
with products synthesized abiotically in the absence of organic
matter. Incorporation of growth media constituents (e.g., K, Fe,
Zn, Cu, Co, Mo) into the biogenic MnOx may also be responsible
for differences relative to the acid birnessite sample.
Effect of Mn-Oxidation on GB-1 Adhesion
and Biofilm Growth
GB-1 cells adhere effectively to ZnSe, Ge, and CdTe surfaces
to form intact biofilms prior to Mn-oxidation. Furthermore, subsequent oxidation of Mn(II) by the intact biofilms bound to the
crystal surfaces did not diminish adhesion once the biofilms were
formed. Adhesion is likely mediated by favorable bonding interactions (e.g., cationic proteins, hydrogen bonding, hydrophobic
interaction, surface roughness) between bacterial biomolecules
and the substrata that overcome the repulsion induced by net
negative charge of bacterial and crystal surfaces (Marshall et al.
1971; Truesdail et al. 1998; Deo et al. 2001; Appenzeller et al.
2002).
Importantly, GB-1 adhesion to negatively-charged crystal
surfaces was negligible for cells encrusted with biogenic MnOx .
These results indicate that bacterial surface chemistry is altered
significantly by the presence of surficial biogenic precipitates.
Given that the point of zero net charge for birnessite is 1.5 to
2.5 (Sposito 1989), it is expected to carry a net negative charge
at the experimental pH (7.4). The coating of bacterial surface
macromolecules by negatively charged MnOx and, in particular, the indication that MnOx interacts preferentially with the
extracellular protein components of the GB-1 surface, suggests
that the MnOx binds these cell surface proteins and diminishes
their availability for subsequent adhesion to negatively charged
surfaces. Since most silicate surfaces are negatively charged, environmental mobility in geomedia is expected to be significantly
enhanced for MnOx -coated cells.
CONCLUSIONS
FTIR spectroscopy with several modes of sample introduction was useful for monitoring compositional changes of cells
before, during and after biologically catalyzed oxidation of
Mn(II). An increased contribution to the FTIR spectra of surficial
proteins is associated with Mn-oxidation during production of a
poorly-crystalline Mn(IV) phase. Changes in protein (amide) regions of the spectra result from protein-MnOx interaction and/or
conformational and compositional changes associated with Mn
oxidation. The apparent surface protein-biogenic MnOx association greatly reduces cell adhesion to negatively charged substrata in aqueous environments. GB-1 cells adhere effectively to
negatively charged ZnSe, Ge, and CdTe crystal surfaces prior
to biogenic MnOx formation to produce intact biofilms. These
biofilms exhibit the capability for effective Mn(II) oxidation,
which then results in an accumulation of biomass at a faster rate
than in the absence of Mn(II). However, planktonic GB-1 cells
exposed to Mn(II) in the absence of crystal surfaces develop biogenic MnOx coatings that preclude their subsequent adhesion to
217
a negatively-charged ZnSe IRE. The data suggest an important
role of surficial proteins in bacterial adhesion to both the experimental crystal surfaces and also to biogenic MnOx .
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